1. Field of the Invention
The invention relates to a method for operating a device for supplying a liquid additive.
2. Related Art
Such devices are used in particular in the automotive sector for the purposes of feeding a liquid additive (fuel, water, ammonia, etc.) to an exhaust-gas treatment device. In particular for the purification of exhaust gases of diesel internal combustion engines, use is made of exhaust-gas treatment devices in which nitrogen oxide compounds in the exhaust gas are converted to non-hazardous substances (nitrogen, carbon dioxide and water) with the aid of a reducing agent—this is the method of selective catalytic reduction (SCR), such as is known to a person skilled in the art. The reducing agent may, for example, be fed in the form of liquid additive to the exhaust-gas treatment device. In this context, urea-water solution is commonly used as liquid additive. A 32.5% urea-water solution is available under the trade name AdBlue® as additive for exhaust-gas treatment.
Devices for supplying a liquid additive for motor vehicles should be as inexpensive as possible and should be constructed using the smallest possible number of different components.
A problem of typical liquid additives is that they can freeze at low temperatures. AdBlue® freezes, for example, at −11° C. In motor vehicles, such low temperatures may arise in particular during relatively long standstill phases in winter. The freezing process causes a volume expansion of the aqueous additive. The volume expansion can damage the device for supplying the liquid additive. This is a problem in particular with regard to components for delivering the liquid additive (such as for example in pumps).
It is possible to design the device for supplying the liquid additive such that it is not damaged as a result of freezing of the liquid additive. This concept is generally relatively expensive. Another possibility is to evacuate the device for supplying the liquid additive after a stoppage of operation of the internal combustion engine, such that no liquid additive can freeze in the device during a standstill phase. This concept however necessitates further actions after the stoppage of operation, which actions may be associated with noise and place an increased burden on energy stores.